Convective inertial accelerometer with metamaterial thermal structure

Abstract

An accelerometer based on convective thermal transport through a fluid is structured without a solid proof mass. Thermal transport through the fluid is sourced and sensed by thermal elements. The thermal elements are comprised of phononic structures which increase power efficiency for accelerometer operation and provide an increased sensitivity to acceleration vectors. The temperature of the sensing element is a function of the vectored acceleration of the enclosed cavity structure. The accelerometer in embodiments provides an extended range for multi-axis accelerations from excitations such as vibration, shock and gravity. Integration of the accelerometer with CMOS signal conditioning circuitry on the same die is convenient.

Claims

1. A convective accelerometer comprised of one or more heater thermal elements and one or more sensor thermal elements disposed within a cavity, the cavity filled with a fluid providing a convective thermal transport from the heater thermal elements to sensor thermal elements, wherein: each thermal element is comprised of one or more micro-platforms wherein each micro-platform is supported by a plurality of nanowires; each nanowire is partially disposed on the one or more micro-platforms and an off-platform region, the off-platform region at least partially surrounding the micro-platform; one or more of the plurality of nanowires is physically configured with one or more first layers, the one or more nanowire first layers comprised of phononic scattering nanostructures and/or phononic resonant nanostructures; the one or more first layers of the plurality nanowires provides a reduction in the ratio of thermal conductivity to electrical conductivity, and the thermal elements provide a means for creating and sensing a vectored change in thermal transport by free convection through the fluid within the cavity.

2. The accelerometer of claim 1 structurally configured with one or more heater thermal elements comprised of resistive heaters.

3. The accelerometer of claim 1 structurally configured with one or more sensor thermal elements comprised of thermistor devices and/or Seebeck thermoelectric devices.

4. The accelerometer of claim 1 comprised of one or more convective conditioning structures restricting and/or guiding the convective thermal transport, the structures providing improvement in an accelerometer performance parameter.

5. The accelerometer of claim 1 wherein the thermal elements are positioned within the cavity to minimize molecular thermal conduction through the fluid and maximize the convective thermal conduction to sensor thermal elements.

6. The accelerometer of claim 1 wherein the fluid within the cavity is comprised of at least one of N.sub.2, He, Ar, Xe, Ne, Kr, SF.sub.6 and CO.sub.2.

7. The accelerometer of claim 1 structurally configured with a reference device comprised of a thermistor or bandgap diode disposed in the off-platform region providing a measurement of absolute temperature.

8. The accelerometer of claim 1 physically configured to provide one or more of linear acceleration, angular acceleration, and inclination vectors.

9. The accelerometer of claim 1 wherein the one or more first layers of the plurality of nanowires is comprised of one or more of semiconductors silicon, germanium, SiGe alloy, SiC, GaN, bismuth and lead chalcoginides, AsH.sub.3, CoSb.sub.3, metal oxides and binary/ternary alloys thereof.

10. The accelerometer of claim 1 wherein the one or more of the plurality of nanowires is comprised of a second layer, the second layer further comprised of an ALD metal selected from one or more of W, Pd, Pt, Mo, Ni, Al, Ag and Au providing an increased electrical conductivity for said nanowire.

11. The accelerometer of claim 1 wherein the one or more of the plurality of nanowires is comprised of a third layer, the third layer further comprised of a dielectric film providing a means for mechanical stress control and/or electrical isolation.

12. The accelerometer of claim 1 wherein the sensor thermal elements are physically configured to provide a differential signal voltage or a unbalanced signal voltage.

13. The accelerometer of claim 1 wherein a heater element is supplied with a constant power to provide a reference quiescent signal level.

14. The accelerometer of claim 1 wherein the sensing thermal elements have an incremental temperature detection limit ranging from less than 1 microdegree Centigrade to 1 degree Centigrade.

15. The accelerometer of claim 1 wherein the cavity has enclosing structural dimensions ranging upward from 100 micrometers.

16. The accelerometer of claim 1 with internal and external electrical connection structure comprising one or more of wire bonding pads, metal interconnect pads and through-semiconductor vias (TSV).

17. The accelerometer of claim 1 wherein the thermal elements are disposed in a single plane or a plurality of planes.

18. The accelerometer of claim 1 structured from a plurality of bonded wafers including at least one semiconductor-on-insulator (SOI) starting wafer

19. The accelerometer of claim 1 disposed within or near a mobile phone, within a wireless network or within a mounted module.

20. A method for sensing acceleration based on the convective accelerometer of claim 1 and the use of an acceleration analyzer, wherein the method is comprised of a sequence of steps: position the convective accelerometer on a calibrated rate table programmed with known vectored accelerations to obtain a calibration database1 of vectored accelerations; operate the convective accelerometer in an application acceleration environment to obtain a calibration database2 of sensor signal levels. develop a multivariate algorithm to uniquely specify a vectored magnitude accelerations for application accelerometer signals based on calibration database1 and calibration database2; operate the accelerometer in an application environment and obtain database3 comprised of application accelerometer signals. determine the vectored magnitude accelerations corresponding to signal levels contained within database3 using the acceleration analyzer programmed with the multivariate algorithm.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0040] FIG. 1 Depicts a plan view of a prior art thermocouple having phononic nanowire structure.

[0041] FIG. 2A Plan view depicting a single axis linear thermal accelerometer with phononic nanowire structure in accordance with the present teachings.

[0042] FIG. 2B Depicts a cross-sectional view depicting the accelerometer of FIG. 2A

[0043] FIG. 3 depicts a plan view depicting a Coriolis angular rate accelerometer with phononic nanowire structure in accordance with the present teachings.

[0044] FIG. 4A is a schematic illustration of a multi-axis convective accelerometer in accordance with the present teachings.

[0045] FIG. 4B depicts a plan view depicting the multi-axis convective accelerometer of FIG. 4A configured in a circular format.

[0046] FIG. 5A depicts plan view of a multi-axis convective accelerometer configured in a rectangular format in accordance with the present teachings.

[0047] FIG. 5B depicts a cross-sectional view of the accelerometer of FIG. 5A formed from two bonded wafers.

DETAIL DESCRIPTION

[0048] Definitions: The following terms as explicitly defined for use in this disclosure and the appended claims:

[0049] disposed on or disposed in means attached to and/or created within.

[0050] providing means physically configured and/or operated to provide.

[0051] inertial accelerometer means an accelerometer providing a vectored measurement of either linear acceleration and/or angular rate acceleration.

[0052] inertial measurement unit or IMU is a sensor providing a measurement of both a linear acceleration vector and an angular acceleration vector.

[0053] convective accelerometer means an accelerometer wherein vectored accelerations are determined by sensing the convective flow of a convective fluid disposed within a cavity and affected by acceleration.

[0054] convective cavity or cavity in this invention means the hermetic volume containing the thermal elements.

[0055] fluid media, convective fluid or fluid means the gas or liquid within the cavity in thermal contact with the thermal elements.

[0056] thermal micro-platform means a platform supported by nanowires wherein the nanowires provide a conducted thermal isolation with respect to a surrounding support platform.

[0057] thermal element means the heater or the temperature sensor disposed within the cavity and comprised of a micro-platform and supporting metamaterial nanowires.

[0058] heater thermal element means any externally-powered device heating a micro-platform including a resistive heater.

[0059] sensor thermal element refers to a thermal element operated to monitor temperature such as a thermistor or Seebeck thermoelectric device.

[0060] reference sensor refers to a temperature sensor disposed outside the cavity and providing a calibration reference.

[0061] metamaterial nanowire or phononic-structured nanowire means a semiconductor of nano-dimensioned structure physically configured for scattering or resonating phonons thereby providing a reduction in thermal conductivity. The metamaterial nanowire may comprise surface, bulk, and embedded structures for Umklapp and other scattering, Bragg resonance and Mie resonance of phonons

[0062] convective conditioning structure means a physical structure interposed into the convective path within the cavity, deflecting the convective thermal transport in a manner which improves one or more accelerometer performance parameters.

[0063] FIG. 1 depicts the plan view of a sensor thermal element within the accelerometer. In this illustrative embodiment, a single thermoelectric couple is comprised of a p-type semiconductor and n-type semiconductor arm partially disposed on a surrounding support platform 102. The p-type semiconductor comprises micro-platform area 104A, nanowire 103A, interconnection 106A, and electrical contact bonding pad 107A. The n-type semiconductor comprises micro-platform area 104B, nanowire 103B, interconnection 106B, and electrical contact bonding pad 107B. The interconnections 106A and 106B are disposed on the surrounding support platform 102. The nanowires and micro-platform extend into the cavity volume 110. The micro-platform 104A and 104B is incrementally heated with convective thermal transport from the heater thermal element.

[0064] In all embodiments, the thermal conductivity of the nanowires 103A and 103B is advantageously reduced by the physical structuring. In an exemplary embodiment, separation between phononic scattering or resonant structures. In other embodiments, the phononic structure supports a phononic resonance. In all embodiments, the metamaterial nanowire structure lowers the ratio of thermal conductivity to electrical conductivity.

[0065] FIG. 2A depicts an plan view of a single axis linear accelerometer embodiment. The heater thermal element 207 is powered externally through contacts 205 and 206 and heats a bubble within fluid in the immediate vicinity within the cavity 110 bounded by perimeter 104. If the accelerometer is accelerated in a positive x-direction, then the Seebeck thermal elements Q 202 and S 203 are incrementally heated more than thermal elements P 201 204 and R due to convective motion of the heated bubble. If the accelerometer is accelerated in the negative x-direction, then thermal elements P and R are heated more than thermal elements Q and S. Acceleration in the x-axis directions is sensed as voltage from Seebeck thermal elements P, Q, R and S. When the accelerometer is calibrated using a rate table, the magnitude of x-axis acceleration is quantified. The thermal elements are disposed on a first wafer having cavity 110. The remainder of the cavity 110 is formed within a second cap wafer.

[0066] FIG. 2B depicts a cross-sectional view of the accelerometer of FIG. 2A as defined by section a-a. The cavity 110 is bounded by the first wafer configured with a surrounding substrate 102, and an oxide film 210. The cavity 110 is bounded above by substrate 208. A eutectic film 210 is used to bond the two wafers and hermetically seal the cavity. The cross-section a-a includes the heater thermal element 207.

[0067] FIG. 3 depicts a plan view of a Coriolis or angular rate accelerometer embodiment with one heater thermal element indicated by heated area 307 and another heater thermal element with bonding pads 305 and 306. The thermal elements are disposed in cavity 110 bounded by periphery 104 within surrounding support structure 102. The heated area 307, when affected by acceleration in the negative x-axis direction, heats Seebeck thermal elements Q 302 and S 303 along the convective path to sensors P 301 and R 304. With angular rotation .sub.z, 308 about the z-axis, thermal elements Q 302 and P 301 are incrementally heated more than thermal elements S 303 and R 304 due to angular acceleration. Angular rotation +.sub.z, in the opposite direction .sub.z, is sensed as voltage from Seebeck thermal elements P, Q, R and S when the thermal bubble is created by the heater connected with bonding pads 305 and 306. When the accelerometer is calibrated using a rate table, the magnitude of x-axis acceleration is quantified.

[0068] FIG. 4A is a schematic depiction of two-dimensional thermal transport sensed by ten Seebeck thermal sensors (A-J) disposed at the periphery of a cavity heated from a central hot spot 413. In this embodiment, the convective spectrometer provides a two-axis linear accelerometer for x- and y-axis accelerations and a single axis angular accelerometer (+.sub.z, and .sub.z).

[0069] In FIG. 4A a linear acceleration in the +x vector direction, heats sensors at positions E 405 and F 406 more than sensors at positions A 401 and J 402. An acceleration in the x direction incrementally heats sensors at positions A 401 and J 410 more than sensors at positions E 405 and F 406. Similarly, an acceleration in the +y direction incrementally heats sensors at positions G, H and I more than sensors at positions B, C and D. An acceleration in the y direction heats sensors at positions B, C and D more than sensors at positions G, H and I. When the accelerometer is rotated +.sub.z about the +z axis, the acceleration incrementally heats sensors at positions D, E and I, J more than sensors A, B and F, G. When the accelerometer is rotated in the opposite direction .sub.z about the z-axis, then sensors located at A, B and F, G are heated more than sensors located at D, E and I, J.

[0070] FIG. 4B depicts the structure and thermal elements depicted in FIG. 4A more clearly where the heater thermal element 413 is surrounded by sensor thermal elements A-J. Thermal heater element 413 suspended with nanowires over cavity 110 with periphery 104 is contained within surrounding support platform 102. Peltier thermoelectric sensor thermal elements A-J are disposed around the cavity 110 periphery with micro-platforms and nanowires extending into the cavity 110. In embodiments, where less sensitivity is required, the sensor thermal elements may be comprised of thermistors. Where a maximum sensitivity for temperature is required, the Peltier thermoelectric thermal elements are comprised of an array having many thermocouples connected in series. The series connection when optimized provides an increased in signal level and an overall increase in signal to noise ratio for the Peltier array. In most embodiments, the temperature of the surrounding support platform is needed for calibration purposes. This reference sensor is depicted as a thermistor 415 contacted with bonding pads 413 and 414.

[0071] The accelerometer structure of FIGS. 4A and 4B as presented above provides an inertial measurement unit having sensitivity for two linear axes (x and y) and sensitivity for one rotational axis .sub.z.

[0072] The accelerometer of FIGS. 4A and 4B also may provide an inertial measurement unit (IMU) with sensitivity to linear acceleration to movements in all three cartesian axes in additional to sensitivity to rotational acceleration around these same three axes. Reduced sensitivity is obtained for linear acceleration in the +/z-directions by sensing the average signal level of all 10 sensors without any differential reference. For instance, a uniform overall reduction in all sensor levels is affected by a linear z-axis acceleration. Sensitivity is obtained for rotational acceleration .sub.z around the x-axis wherein the signal levels at sensors B, C and D are differentially affected. Similarly, sensitivity for rotational acceleration .sub.y around the y-axis is provided by the differential signal levels of sensors E and F compared with A and J.

[0073] To obtain a measure of all vectored acceleration amplitudes, a multivariate analysis of signals from the 10 sensor thermal elements is processed by an acceleration analyzer. The accelerometer signals are determined for a range of accelerations using a rate table to obtain a reference calibration database. The accelerometer is placed in service and subjected to application specific accelerations to obtain an application database. An acceleration analyzer processes the reference database and the application database to quantify the vectored amplitudes of the application specific acceleration using multivariate analyses based on one or more of the 10 variables.

[0074] FIGS. 5A and 5B depict the respective plan and cross-section b-b views of the convective accelerometer structurally configured for enhanced inertial response. This embodiment is comprised of 12 Seebeck thermal elements disposed around the periphery 505 of cavities 526, 529 and 529. Heater thermal element 501 with micro-platform/nanowire structure 502 is suspended within the cavity. All thermal elements are partially supported by the surrounding support platform 530 and dielectric insulator film 525.

[0075] In FIG. 5A the convective transport path terminating into the four remote Peltier sensor thermal elements 513, 519, 530 and 531 is modified by flow conditioning structures 502 in FIG. 5A. In FIG. 5B these convective flow conditioning structures are depicted as 528 with surfaces 523 and 524. These structures 502 reduce the cross-sectional area for thermal transport in the convective paths depicted as 520 and 521. These structures partially block thermal transport to the sensor thermal elements 511, 519, 531 and 532 and reduce the sensed signal level. In this embodiment, structures 502 increase the threshold level at which convective thermal saturation occurs thereby increasing the upper range for acceleration sensing. In other embodiments, the cavity may be shaped to provide a channel of decreasing cross-section to concentrate convective flow into a sensor thermal element thereby increasing sensitivity to an acceleration vector.

[0076] Sensing of accelerations in a lower range is provided by 8 of the 10 sensor thermal elements (including elements 510, 515) disposed symmetrically around the periphery of cavity 526, 529. The total of 10 sensor elements permit sensing of up to 6 acceleration axes including an extended range for linear accelerations for vectored axes x and y. This accelerometer is operated in a similar manner to that disclosed for the accelerometer of FIG. 4.

[0077] The accelerometer in cross-sectional view of FIG. 5B is comprised of two structural support wafers 527 and 530 bonded together with eutectic metal, adhesive such as epoxy, or in some embodiments, direct wafer to wafer bonding. In the exemplary embodiment, the lower wafer is formed of a starting silicon SOI wafer and the upper capping wafer is formed of a starting silicon wafer.

[0078] In embodiments, the accelerometer is formed of semiconductor, ceramic, and glass wafers. In embodiments, the accelerometer if formed of surrounding support structures formed by 3-D additive printing technology.

[0079] In some other embodiments, nanowires 101 are physically created in situ by thin film deposition and annealing processes. These synthesis processes use appropriate precursors and specialized thermal annealing to form nanowires with mesoporous or clustered semiconductor phononic scattering structures comprised of one or more semiconductor material

[0080] In embodiments, the metal layer increases the electrical conductivity of the nanowire and is created by sputtering or evaporative deposition to provide a film, generally an ALD film. FIG. 3B depicts a nanowire 101 physically configured with a dielectric layer 106 sandwiched between an overlying metal film 105 and the device layer 300 of the starting wafer. The dielectric layer in some embodiments includes Si.sub.3N.sub.4 obtained by a CVD process using NH.sub.3 and SiH.sub.4 as precursors. In other embodiments, the dielectric layer 106 is SiO.sub.2 obtained by using a oxide target with RF sputtering. In other embodiments, the dielectric layer 106 is a film of Al.sub.2O.sub.3 obtained using a CVD deposition or reactive sputtering process

[0081] The Seebeck thermoelectric sensors are formed into the silicon device layer by creating alternate heavily doped p- and n-type regions using typically SOG-based dopant with boron and phosphorus.

[0082] Next, level1 vias 1310, 1320 and 1330 and level2 vias 1340 and 1350 are created. The vias are typically formed using a combination of DRIE etching and electroless- or electro-plating of a conductor such as Cu over a thin adhesion layer. Cavities 108 are etched from the backside with cavity areas defined by patterned film such as silicon dioxide or a metal such as Cd. Next, wafers are bonded together using an epoxy, metal, direct bonding process. In this embodiment, solder bumps 1360 are created by electroplating the bonded wafers to provide a flip-chip accelerometer 1300 for soldering directly to a printed circuit board. In other embodiments, contact pads much smaller than solder bumps are used in order to reduce the accelerometer footprint.

[0083] Although various embodiments of this invention have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of this invention. It is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative only of particular embodiments and not limiting. Changes in detail or structure may be made without departing from the basic elements of the invention as defined in the following claims.